LCOS can be thought of as one large liquid crystal formed on a silicon wafer. The silicon wafer is divided into an incremental array of tiny plates. A tiny incremental region of the liquid crystal is influenced by the electric field generated by each tiny plate and the common plate. Each such tiny plate and corresponding liquid crystal region are together referred to as a cell of the imager. Each cell corresponds to an individually controllable pixel. A common plate electrode is disposed on the other side of the liquid crystal.
The drive voltages are supplied from plate electrodes on each side of the LCOS array. In the presently preferred LCOS system to which the inventive arrangements pertain, the common plate is always at a potential of 8 volts. Each of the other plates in the array of tiny plates is operated in two voltage ranges. For positive pictures, the voltage varies between 0 volts and 8 volts. For negative pictures the voltage varies between 8 volts and 16 volts.
The light supplied to the imager, and therefore supplied to each cell of the imager, is field polarized. Each liquid crystal cell rotates the polarization of the input light responsive to the RMS value of the electric field applied to the cell by the plate electrodes. Generally speaking, the cells are not responsive to the polarity (positive or negative) of the applied electric field. Rather, the brightness of each pixel's cell is generally only a function of the rotation of the polarization of the light incident on the cell. As a practical matter, however, it has been found that the brightness can vary by about 5% between the positive and negative field polarities for the same polarization rotation of the light. Such variation of the brightness can cause an undesirable flicker in the displayed picture.
In the case of either positive or negative pictures, as the field driving the cells approaches a zero field, corresponding to 8 volt, the closer each sell comes to white, corresponding to a full on condition. Other systems are possible, for example where the common voltage is set to 0 volts. It will be appreciated that the inventive arrangements taught herein are applicable to all such positive and negative field LCOS imagar driving systems. Pictures are defined as positive pictures when the voltage applied to the common plate electrode a greater than or equal to the largest possible value in the range of the variable plate voltages in the array of the other electrode. Conversely, pictures are defined as negative pictures when the voltage applied to the common plate electrode is less than or equal to the smallest possible value in the range of the variable plate voltages in the array of the other electrode. The phrase “plate voltages” as used herein refers to source voltages applied to plate electrodes of the LCOS array. The designation of pictures as positive or negative should not be confused with terms used to distinguish field types in interlaced video formats.
It is typical to drive the imager of an LCOS display with a frame-doubled signal by sending first a normal frame (positive picture) and then an inverted frame (negative picture) in response to a given input picture. The generation of positive and negative pictures ensures that each pixel will be written with a positive electric field followed by a negative electric field. The resulting drive field has a zero DC component, which is necessary to avoid the image sticking, and ultimately, permanent degradation of the imager. It has been determined that the human eye responds to the average value of the brightness of the pixels produced by these positive and negative pictures.
The present state of the art in LCOS requires the adjustment of the common electrode voltage, denoted VITO or sometimes VCOM, to be precisely between the positive and negative field drive for the LCOS. The balance is necessary in order to minimize flicker, as well as to prevent a phenomenon known as image sticking.
In the prior art it is often tricky to properly bias the common electrode in an imager. Usually, it is done by guesswork. As noted above, when the bias voltage is not optimal there can be image sticking, flicker, and in extreme cases, damage to the imager. Typically, the dynamic range of the positive and negative pictures is chosen and Vito is biased half way between them. This undesirably ignores the details of the gamma correction tables, non-linearity in the anolog circuts, and drift with temperature and age.